An analog angle sensor such as a resolver is conventionally used as an angle sensor to control an AC servo motor or the like. These sensors send angle information with an analog waveform to a motor control apparatus. Therefore, in order for control software in the control apparatus to use this angle information as a motor angle/speed value, processing of converting the angle information to a digital value is required.
FIG. 8 illustrates a conventional common motor drive method (in the case of a resolver sensor) using an analog angle sensor. Furthermore, FIG. 9 illustrates a method of converting an analog angle sensor waveform to a digital waveform in the motor drive method in FIG. 8. As shown in FIG. 9, according to the conventional method, angle information detected and generated by an angle sensor 82 is sent to a motor control apparatus 83 (servo driver) with an analog signal, that is, an angle waveform. Such an analog signal (Sin/Cos signal) is sent to an RID conversion circuit 85 through an interface 84 of a motor control apparatus 83, where it is converted to a digital signal. The digital position information generated is processed by a motor control digital signal processor (DSP) 86, a PWM output is generated and a motor drive circuit 87 is thereby controlled and a motor U/V/W output is fed back to the motor 81. FIG. 9 shows an example of converting angle information of 2048 ct/360° from analog to digital.
In this digital conversion, micro distortion of the analog waveform may cause variations in the count interval of digital values even when the amount of angular movement is the same. This variation is defined as a “pitch error.”
FIG. 10 illustrates the reason that a pitch error occurs.
In the figure, an analog angle sensor waveform rotating at a constant speed is shown in the upper section, and its micro portion is shown in the bottom section. Furthermore, in the bottom left is the ideal waveform, and in the bottom right is the actual one.
As shown in the figure, When a motor is rotating at a constant speed, if the rotation speed is exactly constant and the analog waveform detected by the analog angle sensor is ideal, digital angle information with a constant count interval=pitch should be obtained by position counting of digital conversion processing as shown at the bottom left of the figure. However, micro distortion actually constantly occurs in the analog waveform and this is reflected and results in digital angle information with a variation in the count interval=pitch as shown at the bottom right of the figure.
A pitch error generated in this way causes a “rampage” in the motor current during motor control.
FIG. 11 illustrates a conventional common motor speed calculation method carried out by control software of the motor control apparatus.
Description of symbols is as follows:    Tc: one-count time (sec)    Tsv: speed calculation period (sec)    Nct: number of counts within speed calculation period    Necd: sensor resolution (number of counts in one revolution of motor).
In the figure, Expression <1> shows a calculation expression based on a count and Expression <2> shows a calculation expression based on a one-count time. As shown in the figure, the conventional motor control apparatus generally uses a method of calculating motor speed using angle information which is a digital value based on a count of the angle information within a certain time or one-count time.
Therefore, when there is a pitch error, the count or one-count time vanes and as a result, the calculated motor speed value is not stable and fluctuates, and the above described rampage occurs when the motor speed is controlled, resulting in a problem that it is not possible to improve control response performance or the like. Therefore, attempts have been conventionally made to reduce or prevent influences of rampage using measures such as reducing a control gain.
Hereinafter, this rampage will be defined as a “current ripple.”
On the other hand, improving the accuracy of the angle sensor is essential to improvement of accuracy of motor control and a plurality of technical proposals are conventionally presented and the technique disclosed in Patent Document 1 which will be described later is one of those proposals. This technique samples a plurality of analog signals from a resolver and calculates error parameters for correcting an offset, phase error and amplitude difference using a predetermined calculation expression and corrects a rotation angle value based on the parameters.    [Patent Document 1]: Japanese Patent Application Laid-Open No. 2007-33412 “Error Parameter Extraction Apparatus for Position Detector and Position Detector Having Error Correction Function”.